1. Field of the Invention
The present invention is broadly concerned with improved extrusion systems which include an extruder barrel and screw assembly, together with a product treatment assembly designed for post-extrusion, superatmospheric pressure treatment of extrudates. More particularly, the invention is concerned with such systems and corresponding methods where, in preferred forms, a selectively adjustable back pressure valve assembly is provided upstream of an extrusion die, with a post-extrusion sealed housing for extrudate treatment. The housing may be directly coupled to the extruder assembly so that extrudate passes from the die into the housing; alternately, an atmospheric pressure, product-directing cowling may be employed with a downstream sealed housing.
2. Description of the Prior Art
Extrusion cooking systems have long been used for the preparation of human foods and animal feed products. Broadly speaking, such extrusion systems include an elongated extruder barrel with one or more elongated, axially rotatable, helically flighted extruder screws within the barrel, together with a downstream restricted orifice extrusion die. In typical processing, the feed ingredients are fed into and through the extruder barrel where they are subjected to increasing levels of heat, pressure and shear in order to at least partially cook the ingredients and form an extrudate. This extrudate may be cut or otherwise subdivided at on downstream of the die. Thereafter, the subdivided extrudate is often subjected to post-extrusion treatments such as surface application of fats and drying.
Another post-extrusion treatment which has long been practiced involves passing the extrudate into and through a superatmospheric pressure treatment chamber, which often involves injection of high pressure steam into the chamber so as to establish and maintain the desired superatmospheric pressure conditions therein. Such post-extrusion pressure treatment has been found to effectively condition the extruded products and improve the quality thereof. Thus, U.S. Pat. No. 4,039,691 describes a process wherein food-grade materials are extruded and then directly passed into an elongated superatmospheric pressure chamber while steam is injected into the chamber. In order to maintain superatmospheric pressures within the post-extrusion chamber (e.g., 60-80 psi), the ""691 patent describes the use of spring-loaded or rotary valves, or a rotary letdown pump; more generally, the patent describes the use of any device which allows product to exit the confined post-extrusion chamber while maintaining a predetermined back pressure therein.
Similarly, U.S. Pat. No. 4,139,648 employs an upstream extruder with a sealed chamber post-extrusion treatment device, much in the manner of the ""691 patent. Here again, the treatment chamber is designed so as to maintain superatmospheric pressure conditions therein, normally established via steam injection.
U.S. Pat. No. 3,778,522 is yet another variation of this concept, and employs an extruder equipped with a conventional apertured die and a rotary knife; extrusion pressures at the die are about 500 psig or more, and the knife serves to subdivide the extrudate into small pellets or the like. The post-extrusion treatment involves use of an upright tube having a restricted outlet and a steam inlet; in this fashion, the cut extrudate is treated within the tube at high pressures up to 140 psig.
PCT Publications Nos. WO 99/62361 and WO 01/72153 are still further examples of the equipment and techniques disclosed in the aforementioned U.S. patents. Hence, these publications disclose an extruder device which feeds product directly into a superatmospheric pressure treatment chamber. In order to maintain pressure conditions, one or more rotary valves are employed, together with steam injection. The WO 01/72153 publication describes a very complex arrangement wherein the post-extrusion treatment chamber is shiftably supported so that it can be moved to a non-operative position during startup of the extruder or in the event of a process upset. This is deemed to be a very unwieldy device, which is difficult to operate and entails significant operator time and effort.
The present invention provides improved extrusion systems and corresponding methods for the production of a wide variety of extrudates, especially human foods and animal feeds. Broadly speaking, the extruder systems of the invention include an elongated tubular extruder barrel equipped with at least one elongated, axially rotatable, flighted screw within the barrel. A selectively adjustable back pressure valve assembly is operatively coupled to the extruder outlet and comprises structure defining an elongated passageway with an inlet and an outlet, the inlet communicating with the extruder barrel outlet, and the outlet having a restricted orifice die. In addition, the valve assembly has an apertured valve member selectively shiftable relative to the passageway for altering the effective cross-sectional open area presented by the passageway. In this manner, operating conditions within the extruder can be effectively altered or maintained to insure optimum product output. The overall extruder systems further include a post-extrusion product treatment assembly for receiving product after passage through the extruder and back pressure valve assembly. Such a treatment assembly permits superatmospheric pressure treatment of the extruded product, so as to facilitate density control of the product.
In one form of the invention, the treatment assembly comprises a sealed housing equipped with a rotary outlet valve and which is directly coupled to the valve assembly so that product emerging from the latter passes immediately into the chamber without passage through the atmosphere. In another embodiment, the treatment assembly includes an open, atmospheric pressure product-guiding cowling coupled to the valve assembly outlet, together with a downstream sealed, pressurizable treatment housing. In this embodiment, the extruded product passes through the atmosphere and then into the sealed treatment housing.